Magnesium alloy



Oct. 1, 1968 Stress Tons/ in E longation D. J. STRATFORD 3,404,048

MAGNESIUM ALLOY Filed May 11, 1966 Ageing response Tons/in Agelng Temperature C 3244K, 1 14%; M1, 44% j EMW United States Patent 3,404,048 MAGNESIUM ALLOY David James Stratford, Halesowen, Birmingham, England, assignor to Birmetals Limited, Birmingham, Engr 3,404,048 Patented Oct. 1, 1968 time in that the time to reach optimum properties increases as ageing temperatures decrease.

Our investigations have shown that the effectiveness of the present ageing process is dependent upon the presence land a British company 5 of zinc and manganese in the alloy and their relative pro- Fil May 11, 1966, 5 N 549,316 portions. It has been found theoretically that in order to Claims priority, application Great Britain, May 11, 1965, achieve optimum mechanical properties it is required to :757/ 65 form the hardening precipitate Zn Mg Mn which results 9 Clalmsfrom the zinc and manganese present in the alloy. The

10 lrirtlijcll of zirc 1:0 manghalngse it; Zn Mgt lyln is 4.7?1 to 1.

ance a 0y wou ere ore con am zinc an man- ABSTRACT OF THE DISCLOSURE ganese in this ratio. Preferably the zinc content by weight Magnesium alloys containing, in addition to magnesium, is 4.0 to 10% and the manganese content by weight is 2.0 to 30% zinc and 0.2 to 10% manganese are age- 0.8 to 2.0%. The mechanical properties which can be hardened following heat treatment or hot working and achieved in the manganese base ralloys to which the process rapid cooling by ageing in a first stage at temperature of in accordance with the present invention is applied will {0 for 3 31 hours and in a Second Stage be readily appreciated from the results of tests given by at a temperature of 120 C. to 270 C. for 1 to 70 hours. way of example b l The resulting alloys are greatly improved in their P Table I below shows the results obtained from samples stress and tensile strength as compared to such alloys that O, P Q R, S n T which contained Zinc, 1 15% are aged by Prior known methods manganese with the balance, excluding impurities, magnesium. Each sample was extruded from a 12 inch diam- The present invention consists in a process of ageeterl g biliet iilto a i i g g i hardening an alloy of magnesium containing, in addition 25 C00 6 1 ohroom g m g u Ion ea g; to incidental impurities, 2.0 to Zinc and 0.2 to 10% for 1/2 ours at an t en water queue e manganese after solution heat treatment or hot working Sample 0 was tested lmmedlatqly after the quench and rapid cooling, comprising ageing the alloy in two sample P was tested after ageing only at Is 0 C. after stages, in a first stage of which the alloy is aged within quenching and samples to T were aged, immediately a lower temperature range and in a second stage of which 30 after quehehlhg, for Varymg PerlodS at and then the alloy is aged within a higher temperature range. aged at for 16 hours- TABLE I tats tits/iii with; $322353 0 soiution h t treated and water quenched 1 0 7 1 7 7 P As Sample 0 plus 16 hours at 180 C 20. 4 21. 1 23. 7 10 AS samgie 0 plus 4 hours at 90 C.plus 16 hours 21. 5 25. 8 R Ag sgii g'o plus 8 hours at 90 C.p1us 16 hours 22.1 22. s 25.1 9 s As sggplgb plus 24 hours at 90 0. plus 16 hours 21. 9 23.0 .25. o 9 T L Asgzii riplgb plus 31 hours at 90 0. plus 16 hours 22. 1 23. 1 25. 2 6

at 180 c.

By the ageing process according to the present inven- The Percentage ilohgatioh given in the results and those tion it is possible to achieve better mechanical properties b l e o 4 /A where A is the, initial cross-sectional in an alloy of the kind described than are possible by area of th st piece, normal, single stage, ageing processes. It is also possible The results in the table show that the proof stress and to Obtain equivalent and better Values than those Which tensile strength of the samples treated in accordance with can be Obtained y conventional ageing in pp y the present invention are notably higher than for those less time. which can be reached by ageing in the usual way. It will be The ageing process is applicable to the mag s base observed that the tensile strength achieved notably high alloys in cast or wrought form. As in the case of a single v l fter ageing in the fir t stage for only 4 hours at stage ageing process it is essential that hot working or 90 C, solution heat treatment of the alloy should be followed Whe the ageing temperature of the second stage i immediately afterwards by rapid cooling because the me- 180 C, the lloy i referably maintained at that tem- Chan c l pr per s Which can he achieved y the Present perature for approximately 16 hours, as has been the case ageing process are influenced y the rate f ng after in the examples given above. However when an alloy is hot working or solution heat treatment in that improved d i th fi t stage t 90 C, and the at 180 C. in the Properties result from high Cooling The desired second stage it is possible to achieve noteworthy mechani- 1rapid Cooling y achieved y quenching in Water or cal properties after ageing for as little as 4hours at 180 C. even cooling in air, depending upon the cross-sectional as ill b Shown l t r, dimensions of the alloy article. With the magnesium base alloys to which the two stage Tests have shown that the greater the difference ageing process herein is disclosed is applied, it is possible tween the ageing temperatures of the two stages when the to obtain high mechanical properties by subjecting the ageing tempeltafllfe of one 0f t Stages is Unchanged, alloys to a normal, single stage, ageing but lengthy periods Preferably the ageing temperature of the Second Stage, of time are required. Such mechanical properties and the better are the mechanical proper e Which c be better can be attained in considerably less time 'by ageing achieved. in two stages. This is illustrated by the graph shown in It has been observed that the mechanical properties FIGURE 1 of the accompanying drawings which shows improve as the temperature of ageing in each f t W0 the efiect of the second stage ageing temperature on the stages is lowered. As in a normal, single stage, ageing process there is a relationship between temperature and maximum mechanical properties which can be attained in a magnesium alloy with two stage ageing and, for comparison, the efiect of ageing at those temperatures in a single stage ageing process. The curves on the graph were plotted from results obtained from samples which contained 6.2% zinc, 1.13% manganese with the balance, apart from impurities, magnesium. Each sample was in the 6.23% zinc and 1.41% manganese with the balance, exeluding impurities, magnesium. Each sample before ageing was extruded from a 12 inch diameter billet to a 2 inch diameter bar at 380 C. in a container at 380 C. in a container at 380 C. at 2 feet/minute, solution heat O form of a 2 inch diameter bar which before ageing was treated for 2 hours at 420 C. and then water quenched.

TABLE 111 Sample Condition .1% PS, .2% PS, T.S., Percent tons/in. tons/in. tons/in. elongation U Aged for 24 hours at 90 C. plus 16 hours at 180 C. 22. 1 22. 8 25. 7 V.... Aged for 24 hours at 60 0. plus 16 hours at 180 C. 22. 23. 2 25. 4 7 W Aged for 24 hours at 90 C. plus 32 hours at 120 C. 22. 9 23. 7 25.0 6 X. Aged for 24 hours at 60 C. plus 32 hours at 120 C 23. 9 24. 4 26. 0 5 Y Aged for 24 hours at 90 C. plus 16 hours at 150 C 23.8 24. 6 25. 8 6 Z.- Aged for 24 hours at 60 0. plus 16 hours at 150 C 24. 0 24. 8 26. 1 7

solution treated for 2 hours at 420 C. and then water quenched. In the graph the curve of .1% proof stress for the single aged samples is represented by a broken line, the curve of tensile strength is represented by a dotted line and the curves of .1% proof stress and tensile strength for the samples aged in two stages are represented respectively by a full line and a chain-dotted line. The times in hours to reach maximum values at particular temperatures are indicated on the .1% proof stress curves. On the two stage ageing curves the times given are the total of the times in the two ageing stages as given in the following Table 11:

TABLE II Total time of ageing=ageing in first stage+ageing in second stage:

86 hours=16 hours at 60 C.+70 hours at 120 C. hours=4 hours at 90 C.+16 hours at 150 C. 8 hours=4 hours at 90 C.+4 hours at 180 C. 6 hours (1)=4 hours at 90 C.+2 hours at 210 C. 6 hours (2)=4 hours at 90 C.+2 hours at 240 C. 4 hours=3 hours at 90 C.+1 hour at 270 C.

The etfect of ageing temperature on the percent elongation of the alloy with single and two stage ageing is shown by curves Y and Z respectively of the graph.

It will be seen from the graph of FIGURE 1 that whereas, in the temperature range in which the alloy sample was aged, it was necessary with single ageing to age for 142 hours ot reach the maximum possible values for .1% proof stress and tensile strength, similar values were reached with the two stage ageing processin lessthan 20 hours and even higher values were obtained by two stage ageing in a shorter time than was necessary to obtain the maximum properties by single ageing. The above Table II and There is an improvement in the mechanical properties as the proportion of zinc and manganese which is in the ratio 4.75 to 1, as theoretically required to form the hardening precipitate Zn Mg Mn, increases. The linear form of the ageing response curve in relation to alloys containing increasing amounts of zinc and manganese in this ratio is shown clearly by the graph of FIGURE 2 of the accompanying drawings. The curves of the graph of FIG- URE 2 were drawn from the results obtained from samples which after solution heat treatment at 420 C. were aged for 24 hours at 90 C. and then for 16 hours at 180 C. The curve of tensile strength in the graph is represented by a full line, the .1% proof stress curve is represented by a chain-dotted line and the .2% proof stress curve is represented by a broken line.

The effect of the ageing process according to the present invention has so far been exemplified only in relation to the magnesium base alloys in wrought form. It is possible, however, to obtain high mechanical properties in cast alloys by two-stage ageing. This is shown in Table IV by the results which were obtained from cast samples Ac and B0 containing by weight 5.46% zinc and 1.1% manganese with the balance, excluding impurities, magnesium. The samples were grain refined using hexachloroethane, sand cast into British D.T.D. Aircraft Standard test bars from which 0.64 inch diameter tensile test pieces were machined, solution treated for 12 hours at 340 C. plus 4 hours at 370 C. and plus 8 hours at 420 C., and then in the case of sample Ac air cooled and in the case of sample Bc quenched in boiling water. Each sample .was tested immediately after cooling from the solution heat treatment and again after it was subjected to an ageing process consisting of 30 hours at 60 C. followed by hours at 120 C.

TABLE IV Sample Condition .1% PS, .2% PS, T.S., Percent tons/in. tons/in). tons/in. elongation Air cooled after solution treatment 4. 3-4. 6 5-5. 2 11. 5-12. 5 6-8 Air cooled after solution treatment plus 30 hours 8-8. 6 8. 9-9. 1 11. 3-11. 8 2-3 at C. plus 50 hours at 120 C. Boiling water quenched after solution treatment 4-4. 5 4. 8-5 11-12 6-8 Bo Boiling water quenched after solution treatment 10-10. 5 11-11. 5 13. 5-14 2-3 plus 30 hours at 60 C. plus 50 hours at 120C.

the graph of FIGURE 1 further show that the lower the temperature of ageing in each of the two stages the higher are the mechanical properties which can be achieved although the time to reach the optimum is increased.

The mechanical properties are also influenced by the difference between the ageing temperatures of the two stages in that the greater the difference between the two ageing temperatures, when one of the temperatures is unchanged, the better are the mechanical properties which can be obtained. This improvement in properties is most apparent when the temperature of the higher temperature ageing stage is maintained constant and the temperature of ageing in the lower temperature stage is varied to increase the temperature difference between the two stages. The effect of the temperature difference will be seen from Table III below in which the results are given of tests made on samples, U, V, W, X, Y and Z which contained I claim:

1. A process of age-hardening a magnesium base alloy after solution heat treatment, or hot working, and rapid cooling, which comprises ageing an alloy which consists of by weight 2.0% to 30% zinc and 0.2% to 10% manganese with the balance, apart from impurities, magnesium, in a first stage in which the alloy is aged for 3- to 31 hours at a substantially constant temperature in the range 60 C. to C. and ageing said alloy in a second stage in which, following directly after the first stage, the alloy is aged for 1 to 70 hours at a substantially constant temperature in the range C. to 270 C.

2. A process of age-hardening a magnesium base alloy after solution heat treatment, or hot working, and rapid cooling, which comprises ageing an alloy which consists of by weight 4.0% to 10% zinc and 0.8% to 2.0% manganese with the balance, apart from impurities, magnesium, in a first stage in which the alloy is aged for 3 to 31 hours at a substantially constant temperature in the range C. to 90 C. and ageing said alloy in a second stage in which, following directly after the first stage, the alloy is aged for 1 to hours at a substantially constant temperature in the range 120 C. to 270 C.

3. A process according to claim 2 wherein the alloy is aged in the first stage for 24 hours at C. and in the second stage for 32 hours at C.

4. A process according to claim 2 wherein the alloy is aged in the first stage for at least 4 hours at 90 C. and in the second stage for at least 16 hours at C.

5. A process according to claim 2 wherein the alloy is aged in the first stage for at least 3 hours at 90 C. and in the second stage for 1 hour at 270 C.

6. A process according to claim 2 wherein the alloy is aged in the first stage for at least 16 hours at 60 C. and in the second stage for from 32 to 70 hours at 120 C.

7. A process according to claim 2 wherein the alloy is aged in the first stage for at least 24 hours at 60 C. and in the second stage for at least 16 hours at 180 C.

8. A process as claimed in claim 2 wherein said alloy contains by weight 4.0% to 10.0% zinc and 0.8 to 2.0% manganese with the balance, apart from impurities, magnesium and in which alloy the ratio of zinc t-o manganese is 4.75 to 1.

9. A magnesium base alloy which contains by weight 4.0% to 10% zinc and 0.8% to 2.0% manganese with the balance apart from impurities, magnesium, and the ratio of zinc to manganese in the alloy being 4.75 to 1, prepared by a process as defined in claim 8.

References Cited UNITED STATES PATENTS 1,886,251 11/1932 Gann 75168 1,936,550 11/1933 Gann et al. 148--161 2,275,188 3/1942 Harrington 14812.7 2,477,503 7/1949 Vargo 148161 CHARLES N. LOVELL, Primary Examiner. 

